Process for manufacturing thin sheets made of 7XXX aluminum alloy suitable for shaping and assembly

Abstract

The invention relates to the process for manufacturing a rolled product based on an aluminum alloy, in particular for the automotive industry wherein, successively, a bath of liquid metal made from an aluminum-based alloy. The invention also relates to the products obtained by this process and the use thereof for the manufacture of a motor vehicle.

The invention claimed is: 
     
       1. A process for manufacturing a rolled product based on an aluminum alloy, optionally for the automotive industry wherein, successively,
 a) making a bath of liquid metal from an aluminum-based alloy comprising 4 to 7% by weight of Zn, 1.0 to 3.0% by weight of Cu, 1.5 to 3.5% by weight of Mg, at most 0.50% by weight of Fe, at most 0.40% by weight of Si, at least one element chosen from Zr, Mn, Cr, Sc, Hf and Ti, the amount of said element, if it is chosen, being 0.05 to 0.18% by weight for Zr, 0.1 to 0.6% by weight for Mn, 0.05 to 0.3% by weight for Cr, 0.02 to 0.2% by weight for Sc, 0.05 to 0.5% by weight for Hf and 0.005 to 0.15% by weight for Ti, the other elements being at most 0.05% by weight each and 0.15% by weight in total, the remainder being aluminum, 
 b) casting a rolling ingot from the bath of liquid metal, 
 c) optionally, the rolling ingot is homogenized, 
 d) hot rolling and optionally cold-rolling the rolling ingot to give a sheet, 
 e) solution heat treating and quenching the sheet, 
 f) optionally, leveling the sheet and/or subjecting to controlled traction with cumulative deformation of at least 0.5% and less than 3%, 
 g) performing heat treatment wherein the sheet reaches a temperature of between 6° and 120° C. for 8 to 16 hours, 
 h) aging the sheet for at least 30 days at ambient temperature. 
 
     
     
       2. The process according to  claim 1  wherein the thickness of the sheet is between 0.4 and 4 mm after the hot rolling and after the optional cold rolling. 
     
     
       3. The process according to  claim 1  wherein during the heat treatment in g) the sheet reaches a temperature of between 8° and 100° C. for 10 to 14 hours. 
     
     
       4. The process according to  claim 1  wherein the heat treatment in g) is performed at the outlet of a continuous solution heat treatment and quenching line, with the sheet being re-heated to a sufficient temperature to ensure that, after a coiling, the sheet reaches a temperature of between 6° and 120° C. for 8 to 16 hours. 
     
     
       5. The process according to  claim 4  wherein the sheet is cooled after quenching to a temperature of between 2° and 40° C. and re-heated to a temperature of between 7° and 90° C. then cooled slowly to ensure that the temperature is maintained at a temperature of at least 60° C. for at least 10 hours. 
     
     
       6. The process according to  claim 1  wherein quenching is performed using water, the temperature of which is between 2° and 60° C. 
     
     
       7. The process according to  claim 1  wherein said alloy is one of AA7010, AA7012, AA7022, AA7122, AA7023, AA7032, AA7033, AA7040, AA7140, AA7050, AA7050A, AA7150, AA7250, AA7075, AA7175, AA7475. 
     
     
       8. The process according to  claim 1  wherein the change in R p0.2 (TL) during h) is less than 15 MPa. 
     
     
       9. The process according to  claim 1  wherein after h),
 i) a shaping operation is performed on said sheet with local deformation reaching at least 2%, 
 j) the shaped sheet is assembled on a body in white motor vehicle, optionally by welding or riveting, 
 k) baking is performed wherein the sheet reaches a temperature of between 16° and 200° C. for 15 minutes to 1 hour. 
 
     
     
       10. The process according to  claim 9  wherein the shaping operation is performed by stamping at a temperature of between 15° and 250° C. 
     
     
       11. The process according to  claim 9  wherein the shaping operation is performed by profiling or bending or stamping at ambient temperature. 
     
     
       12. The process according to  claim 1  wherein the aluminum-based alloy comprises 5.5 to 6.2% by weight of Zn. 
     
     
       13. The process according to  claim 1  wherein the aluminum-based alloy comprises 1.2 to 2.0% by weight of Cu. 
     
     
       14. The process according to  claim 1  wherein the aluminum-based alloy comprises 2.2 to 3.0% by weight of Mg. 
     
     
       15. The process according to  claim 1  wherein the aluminum-based alloy comprises 0.05 to 0.25% by weight of Fe. 
     
     
       16. The process according to  claim 1  wherein the aluminum-based alloy comprises 0.03 to 0.15% by weight of Si. 
     
     
       17. The process according to  claim 1  wherein no shaping operation is performed on said sheet before step h. 
     
     
       18. The process according to  claim 1  wherein the thickness of the sheet is between 0.8 and 3 mm after the hot rolling and after the optional cold rolling. 
     
     
       19. The process according to  claim 1  wherein the heat treatment in g) is performed at the outlet of a continuous solution heat treatment and quenching line, with the sheet being re-heated to a sufficient temperature to ensure that, after a coiling, the sheet reaches a temperature of between 8° and 100° C. for 10 to 14 hours. 
     
     
       20. The process according to  claim 1  wherein quenching is performed using water, the temperature of which is between 3° and 50° C. 
     
     
       21. The process according to  claim 1  wherein the change in R p0.2 (TL) during h) is less than 10 MPa. 
     
     
       22. The process according to  claim 1  wherein the change in R p0.2 (TL) during h) is less than 7 MPa. 
     
     
       23. The process according to  claim 1  wherein after h),
 i) a shaping operation is performed on said sheet with local deformation reaching at least 2%, 
 j) the shaped sheet is assembled on a body in white motor vehicle, optionally by welding or riveting, 
 k) baking is performed wherein the sheet reaches a temperature of between 17° and 190° C. for 15 minutes to 1 hour.